In the global movement to save energy such as electric power in recent years, there has been strong demand for higher efficiency of electrical equipment. There has also been increasing demand especially for smaller iron core materials, to reduce the size of electrical equipment. Moreover, recent environmental consciousness has raised urgent demand to accommodate recycling of iron core materials of electrical equipment.
Of the above-mentioned demands, higher efficiency of electrical equipment and smaller iron core materials are effectively realized by improving the magnetic properties of an electrical steel sheet which is a material of an iron core. In the field of conventional non-oriented electrical steel sheets, a technique of increasing the content of Si, Al, Mn and the like in steel to increase the electrical resistance and reduce the eddy current loss has been typically used as means to reduce especially the iron loss from among the magnetic properties.
However, that technique has a fundamental problem of inevitably causing a decrease in magnetic flux density.
Techniques of not only increasing the content of Si, Al and the like but also either decreasing C and S or increasing alloy content such as adding B as described in JP S58-151453 A or adding Ni as described in JP H3-281758 A, are also commonly known as a means to reduce eddy current loss.
Those techniques of adding alloy components improve the iron loss property, but are unsatisfactory in that they have little effect in improving the magnetic flux density. Besides, with alloy addition, the steel sheet increases in hardness and decreases in workability. Such a non-oriented electrical steel sheet has poor versatility even when processed and used in electrical equipment, and its use is significantly limited.
Several methods of changing the manufacturing process and improving the degree of integration of crystal orientation in a product sheet, i.e. the texture, to improve the magnetic properties have been proposed. For example, JP S58-181822 A discloses a method whereby steel containing 2.8 to 4.0 mass % Si and 0.3 to 2.0 mass % Al is warm rolled in a temperature range from 200° C. to 500° C. to develop {100}<UVW> texture. JP H3-294422 A discloses a method whereby steel containing 1.5 to 4.0 mass % Si and 0.1 to 2.0 mass % Al is hot-rolled and then subjected to a combination of hot band annealing at 1000° C. or more and 1200° C. or less and cold rolling at a reduction ratio of 80% to 90% to develop {100} texture.
However, the magnetic property improvement effects and in particular the magnetic flux density improvement effects by those methods are still unsatisfactory, and the issues of workability and recyclability remain unsolved. In detail, if Al is contained in steel to a certain extent or more, the hardness of the steel sheet increases, which causes problems such as impairing the workability and, when the iron core material is recycled or scrapped by a user, damaging electrodes of an electric furnace.
The following problem remains, too. In casting a motor shaft or the like using the recycled material of the iron core, if 0.1 mass % or more Al is contained, the molten steel is susceptible to surface oxidation and the viscosity increases during casting, as a result of which the mold-filling ability of the molten steel deteriorates. This hampers a sound casting.
To solve the above-mentioned problems, each of JP 3888033 B where Al content is 0.02% or less, JP 4126479 B where Al content is 0.017% or less, JP 4258951 B where Al content is 0.010% or less, and JP 4258952 B where Al content is 0.030% or less discloses a technique of manufacturing a non-oriented electrical steel sheet with high magnetic flux density and low iron loss by reducing the amount of impurities such as S and N and controlling the average grain size after hot band annealing, the cold rolling conditions or the like.
However, reducing the Al content according to the above-mentioned techniques causes a new problem of poor stability in magnetic properties. Besides, the steel sheet after final annealing clearly has poor surface appearance such as a noticeable whitish stripe pattern or the like.
It could therefore be helpful to provide a hot-rolled steel sheet for production of a non-oriented electrical steel sheet with not only excellent magnetic properties such as iron loss and magnetic flux density but also excellent recyclability and steel sheet surface appearance, and an advantageous method of manufacturing the hot-rolled steel sheet.